Ehv transmission line and shield assembly



June 29,1965 .1. J. LA FOREST EHV TRANSMISSION LINE AND SHIELD ASSEMBLYFiled Sept. 18, 1963 United States Patent 3,192,310 EHV TRANSMISSIONLINE AND SHIELD ASSEMBLY James J. La Forest, Pittsfield, Mass, assignorto General Electric Company, a corporation of New York Filed Sept. 18,1963, Ser. No. 309,832 Claims. (Cl. 174-127) This invention relates toEHV transmission lines and more particularly to improvements in shieldedconductor configurations for such lines.

EHV as used herein covers a range of 287-l000 kilovolts and above. Amajor problem associated with the design of EHV systems is the factthat, as voltages have been increased, the conductor size has had to beincreased in order that the electric field strength perpendicular to thesurface of the conductor be kept low. This increase in conductor sizehas proceeded, in many cases, to the point where the current carryingcapacity of the conductor is far in excess of what is required for theline load. This problem has been met in the past by the bundled orexpanded conductor designs where unneeded metallic cross section isreplaced by air or impregnated paper. In all such arrangaments, the sameelement provides both the current carrying and the corona shieldingfunctions which tends to increase the size and cost of the EHV lines.

In accordance with this invention, the load current carrying functionand the corona shielding function are provided by separate and distinctelements substantially different in kind. Thus the load current carryingfunc tion is provided by a conventional conductor such as an ACSR (foraluminum cable steel reinforced) conductor which has high electricalconductivity, high mechanical tensile strength and which has a crosssection or diameter no greater than is necessary for carrying the loadcurrent. The corona shielding or prevention function is provided by arelatively low conductivity, light weight, low tensile strengthconductor or semi-conductor electrically and mechanically connected tothe current carrying conductor so as to be supported by and have thesame potential as the current carrying conductor while not carrying anyappreciable part of the load current or appreciably increasing theweight of the line.

An object of the invention is to provide a new and improved EHV linemultiple shielded conductor.

Another object of the invention is to provide a novel multiple conductorfor EHV lines in which the current carrying and corona shieldingfunctions are provided by separate elements differing in kind.

The invention will be better understood from the following descriptiontaken in connection with the accompanying drawing and its scope will bepointed out in the appended claims.

In the drawing,

FIG. 1 is an illustration of an embodiment of the invention forupgrading an existing line either as to line voltage or corona loss orradio noise performance,

FIG. 2 is an illustration of a modification suitable for use in newlines especially in the 500 kv. region and up where the foul weatherperformance becomes most important,

FIG. 3 is a further modification showing the different physicalrelationship of the current carrying and corona shielding elements, and

FIG. 4 is another modification in which the current carrying conductoris inside the tubular corona shielding high resistance conductor.

Referring now to the drawing and more particularly to FIG. 1, there isshown therein the three phase conductors 1, 2, and 3 of an existingthree phase EHV transmission line which has been upgraded as to voltageand 3,192,310 Patented June 29, 1965 "Ice corona performance inaccordance with the present invention. Each conductor consists of aconventional line current carrying conductor 4 such as an ACSRconductorhaving a plurality of aluminum strands 5 surrounding a steelcore 6. In order to permit this line to operate at substantially highervoltages without forming corona or producing resulting objectional radiointerference, relatively light weight low conductivity tubes 7 aresuspended below the conductors 4 by electrical conductive supportingmeans 8 which may be in the form of metallic straps or bands. Therelative cross section or diameter of the current carrying conductors 4and corona shielding tubes 7 will be determined by the voltage andcorona performance desired. They are shown by way of example as havingsubstantially equal diameter or cross section. However, the coronashielding tube 7 might be larger in diameter or cross section thanconductors 4. The corona shielding element 7 may be made of extrudableplastic material such as synthetic rubber, butadiene or polyethylenecontaining a predetermined amount of uniformly dispersed filler materialsuch as carbon black or graphite to impart a relatively low electricalconductivity to the tubes 7 in comparison with the conductors 4. Forexample, the conductivity of the tubes 7 may be in the range of 10 to lmhos per meter whereas aluminum typically has a conductivity of 3.6)(10'mhos per meter. By means of the construction shown in FIG. 1 practicallyall of the load current is carried by the conductors 4 but the shieldingelements 8 have the same potential as the conductors 4 due to theelectrical connections provided by the elements 8 so that the electricfield intensity at the surface of the conductor is reduced considerably,thus greatly reducing its propensity to produce corona discharges.

In the modification shown in FIG. 2, the modified multi-part lineconductors 1', 2', 3 each have current carrying conductors 4 essentiallythe same as in FIG. 1 but for higher voltage operation such as 500 kv.and above and particularly for controlling corona under rain or snow orsleet conditions substantially larger diameter shielding conductors 7'are provided, these being shown as having a diameter about four timesthe diameter of the conductors 4. They may, however, be made of the samematerial as the shielding conductor 7 of FIG. 1.

For preventing the accumulation of ice on the shields 7, they may beprovided with a series of embedded longitudinally extending relativelylow resistance conductor filaments 9 which will carry sutficient currentto produce sufficient 1 R heating losses to maintain the temperature ofthe corona shielding members 7' above the melting point of ice.

In the modification shown in FIG. 3, the tubular corona shieldingelement 7" is formed or molded with a longitudinal groove 10 in whichthe conductor 4 is nested so as to form a more intimate physicalrelationship between the conductor and the corona shield, they of coursebeing interconnected by suitable members 8' corresponding in function tothe members 8 in FIGS. 1 and 2. This particular arrangement enhances theelectrostatic shielding properties of member 7".

In the modification shown in FIG. 4, the conductor 4 is inside thecorona shielding element 7" so that interconnecting means such as 8 or8' are unnecessary. However, this arrangement has the disadvantage incomparison with the arrangements of FIGS. 1, 2, and 3 in that theshielding element 7" provides a certain amount of heat insulationsurrounding the conductor 4 so that it will not be as effectively cooledby the atmosphere as in the other figures where the conductor 4 isentirely outside the corona shield. This is an important factor becauseat full load the line con-ductors are quite hot and have a very substantial temperature rise over the ambient air with the result 3 that whenthere are long spans between supporting towers the thermal expansion dueto the high temperature causes substantial sag in the line conductorswhich, of course, materially affects the minimum clearance between theconductors and ground and hence the tower height and the cost of theline.

As shown in FIG. 4, the shielding element 7" may be provided with aplastic zipper 11 for fitting it over the line conductor 4.

It is, of course, not essential that the tubular shielding elementsextend throughout the entire length of the line, and they can be usedselectively along comparatively short lengths of the line such as whenthe line passes near dwellings and where foul weather performance wouldbe objectionable from a radio interference standpoint.

Due to the hollow tubular shape of the conducting shields, they can beused as microwave guides and thus be made to serve the additionalfunction of a communic tion channel between their ends as a supplementto or in lieu of conventional carrier current communication channelswhich use the current carrying conductor.

Another advantage of the invention is that it frees the current carryingconductor from the need for a perfectly smooth surface so that it neednot be deburred or carefully handled to prevent surface marring. Withoutthe shielding conductor, such burrs, points or other irregularities onthe conductor surface provide potent sources of corona, but with theaddition of the tubular shield, the tendency for corona to form isgreatly reduced.

While there have been shown and described particular embodiments of theinvention, it will be obvious to those skilled in the art that changesand modifications may be made without departing from the invention, andtherefore it is intended by the appended claims to cover all suchchanges and modifications as fall within the true spirit and scope ofthe invention.

What I claim as new and desire to secure by Letters Patent of the UnitedStates is:

1. A high voltage transmission line cable assembly comprising anelongated main current carrying conductor of high conductivity electricconducting material having a cross-sectional area of a magnitudedesigned primarily to conduct a predetermined desired load currentwithout excessive heating, a hollow tubular shielding member suspendedfrom said conductor in parallel closely spaced nonconcentric relationthroughout at least a portion of the length of said conductor and havinga conductivity of the order of 1/10 to 1/ 10 times that of saidconductor, said shielding member having :an outer cross-sectional areaat least as large as that of said conductor and a hollow interior freeof high conductivity material over at least a major portion of itsinterior cross-sectional area, and means electrically connecting saidconductor and shielding member together at a plurality of pointsthroughout their contiguous lengths thereby to maintain said conductorand shielding member at substantially equal potentials at adjacentpoints throughout said lengths, whereby said shielding member reducesthe electrostatic field gradient in the region of said conductor whilesaid conductor serves primarily to conduct load current.

2. A cable assembly according to claim 1 in which said current-carryingconductor and said hollow tubular shielding member are disposed inparallel spaced-apart relation and connected at spaced points byelectrically conductive coupling members.

3. A cable assembly according to claim 1 wherein said shielding memberis of substantially circular cross-section and has a radius of the orderof several times the radius of said conductor.

4. A cable assembly according to claim 3 wherein said current-carryingconductor and said hollow tubular shielding member are in substantiallycontinuous lengthwise engagement throughout their contiguous lengths.

5. A cable assembly according to claim 3 wherein said tubular shieldingmember is formed of an extrudable plastic electric insulating materialcontaining a uniformly dispersed filler of comminuted electricconducting material.

References Cited by the Examiner UNITED STATES PATENTS 1,725,120 8/29Williams l74-41 X 2,314,798 3/43 Peterson 174-128 FOREIGN PATENTS396,898 8/33 Great Britain. 800,903 9/58 Great Britain.

OTHER REFERENCES Anaconda Wire and Cable Company Publication, C-78, page4, 1949.

JOHN F. BURNS, Primary Examiner. DARRELL L, CLAY, Examiner.

1. A HIGH VOLTAGE TRANSMISSION LINE CABLE ASSEMBLY COMPRISING ANELONGATED MAIN CURRENT CARRYING CONDUCTOR OF HIGH CONDUCTIVITY ELECTRICCONDUCTING MATERIAL HAVING A CROSS-SECTIONAL AREA OF A MAGNITUDEDESIGNED PRIMARILY TO CONDUCT A PREDETERMINED DESIRED LOAD CURRENTWITHOUT EXCESSIVE HEATING, A HOLLOW TUBULAR SHIELDING MEMBER SUSPENDEDFROM SAID CONDUCTOR IN PARALLEL CLOSELY SPACED NONCONCENTRIC RELATIONTHROUGHOUT AT LEAST A PORTION OF THE LENGTH OF SAID CONDUCTOR AND HAVINGA CONDUCTIVITY OF THE ORDER OF 1-10**4 TO 1/10**8 TIMES THAT OF SAIDCONDUCTOR, SAID SHIELDING MEMBER HAVING AN OUTER CROSS-SECTIONAL AREA ATLEAST AS LARGE AS THAT OF SAID CONDUCTOR AND A HOLLOW INTERIOR FREE OFHIGH CONDUCTIVITY MATERIAL OVER AT LEAST A MAJOR PORTION OF ITS INTERIORCROSS-SECTION AREA, AND MEANS ELECTRICALLY CONNECTING SAID CONDUCTOR ANDSHIELDING MEMBER TOGETHER AT A PLURALITY OF POINTS THROUGHOUT THEIRCONTIGUOUS LENGTHS THEREBY TO MAINTAIN SAID CONDUCTOR AND SHIELDINGMEMBER AT SUBTANTIALLY EQUAL POTENTIALS AT ADJACENT POINTS THROUGHOUTSAID LENGTHS, WHEREBY SAID SHIELDING MEMBER REDUCES THE ELECTROSTATICFIELD PRADIENT IN THE REGION OF SAID CONDUCTOR WHILE SAID CONDUCTORSERVES PRIMARILY TO CONDUCT LOAD CURRENT.